Processes for the preparation of higher molecular weight resins from lower molecular weight liquid epoxy resins are known. See, for example, U.S. Pat. Nos. 3,477,990, 3,547,881, 3,978,027, 3,824,212, and 4,320,222.
Simply, these processes comprise reacting a lower molecular weight polyepoxide (i.e., below about 3500) with a polyhydric phenol in the presence of an etherification catalyst to produce higher molecular weight solid polyepoxides. These solid polyepoxides may then be cured with conventional epoxy resin curing agents such as amines, anhydrides, mercaptans, imidazols, etc. to product coatings, adhesives, castings, etc. Also, these so-called fusion polyepoxides may be reacted with various compounds to produce adducts which are useful as curing agents for other polyepoxides or adducts which may be cured with epoxy curing agents depending upon the relative stoichiometry of the adducting reaction.
Although the above fusion technique is the preferred method for preparing solid polyepoxides, an older method is still used which comprises reacting a polyhydric phenol such as Bisphenol A with epichlorohydrin in the presence of an etherification catalyst followed by a dehalogenation step.
This so-called conventional resin has been utilized in various water borne primer and coating formulations. For example, such solid epoxy resin has been adductd with aromatic acids such as paraaminobenzoic acid (PABA) to produce water-borne primer coatings. See, for example U.S. Pat. Nos. 4,094,844, 4,098,744 and 4,119,609.
It was found that when the solid epoxy resin prepared by the fusion techniques were used in such water borne compositions such as the PABA systems, the resulting formulations do not exhibit viscosities which are acceptable for many applications.
It has now been found that fusion epoxy resins possessing an acceptable viscosity range and increased primary hydroxyl functionality can be prepared by a modified fusion process.